Question: $ E = \left[\begin{array}{rrr}-2 & 5 & 0 \\ 5 & 3 & 2\end{array}\right]$ $ B = \left[\begin{array}{rr}1 & -1 \\ 2 & -1 \\ 4 & 3\end{array}\right]$ What is $ E B$ ?
Solution: Because $ E$ has dimensions $(2\times3)$ and $ B$ has dimensions $(3\times2)$ , the answer matrix will have dimensions $(2\times2)$ $ E B = \left[\begin{array}{rrr}{-2} & {5} & {0} \\ {5} & {3} & {2}\end{array}\right] \left[\begin{array}{rr}{1} & \color{#DF0030}{-1} \\ {2} & \color{#DF0030}{-1} \\ {4} & \color{#DF0030}{3}\end{array}\right] = \left[\begin{array}{rr}? & ? \\ ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ E$ , with the corresponding elements in column $j$ of the second matrix, $ B$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ E$ with the first element in ${\text{column }1}$ of $ B$ , then multiply the second element in ${\text{row }1}$ of $ E$ with the second element in ${\text{column }1}$ of $ B$ , and so on. Add the products together. $ \left[\begin{array}{rr}{-2}\cdot{1}+{5}\cdot{2}+{0}\cdot{4} & ? \\ ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ E$ with the corresponding elements in ${\text{column }1}$ of $ B$ and add the products together. $ \left[\begin{array}{rr}{-2}\cdot{1}+{5}\cdot{2}+{0}\cdot{4} & ? \\ {5}\cdot{1}+{3}\cdot{2}+{2}\cdot{4} & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ E$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ B$ and add the products together. $ \left[\begin{array}{rr}{-2}\cdot{1}+{5}\cdot{2}+{0}\cdot{4} & {-2}\cdot\color{#DF0030}{-1}+{5}\cdot\color{#DF0030}{-1}+{0}\cdot\color{#DF0030}{3} \\ {5}\cdot{1}+{3}\cdot{2}+{2}\cdot{4} & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rr}{-2}\cdot{1}+{5}\cdot{2}+{0}\cdot{4} & {-2}\cdot\color{#DF0030}{-1}+{5}\cdot\color{#DF0030}{-1}+{0}\cdot\color{#DF0030}{3} \\ {5}\cdot{1}+{3}\cdot{2}+{2}\cdot{4} & {5}\cdot\color{#DF0030}{-1}+{3}\cdot\color{#DF0030}{-1}+{2}\cdot\color{#DF0030}{3}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rr}8 & -3 \\ 19 & -2\end{array}\right] $